1. Field of the Invention
This invention relates to a resin composition that has a high degree of cross-linking, an excellent cross-linking speed at normal temperature, and a flat and smooth extrusion appearance. This invention also relates to a wire and a cable using the resin composition.
2. Description of the Related Art
Polyethylene is excellent in an electrical insulation property, thus it is widely used as a coating material of a wire and a cable. In particular, a cross-linked polyethylene obtained by that a cross-linking structure is introduced into polyethylene can be provided with a heat resistance property, so that it is widely used for a coating material of a power cable or the like.
A main cross-linking method of polyethylene includes an organic peroxide cross-linking, an electron beam irradiation cross-linking and a silane cross-linking. In the silane cross-linking, a polyethylene that is graft-copolymerized with a silane compound is formed into a shape and then it is left in a high temperature and a high humidity environment in which water exists, thereby the cross-linking is progressed.
A wire and a cable using the silane cross-linked polyethylene are generally manufactured as follows. First, for the purpose of graft-copolymerizing a silane compound with a polyethylene, a polyethylene, a silane compound, and a free radical generating agent are kneaded so as to react with each other. Next, a silanol condensation catalyst is added thereto, and then the reactant is extruded and coated on a conductor formed of copper or a cable core.
A method for carrying out the above-mentioned process includes a method that has a process in which a polyethylene graft-copolymerized with a silane compound is fabricated, and separately from the aforementioned step, a silanol condensation catalyst and the polyethylene are kneaded so as to be extruded and coated, and a method that has a process in which all the kneading steps are carried out by one extruder and then the reactance is extruded and coated.
According to globalization of market in recent years, a movement toward matching domestic standards with international standards is activated. Even in a low-tension power cable field, JIS (Japanese Industrial Standards) matched to IEC (International Electrotechnical Commission U.S., Inc.) Standard are established, and it is necessary for a resin composition used as an insulation to pass a hot set test at the test condition that an air temperature is 200 degrees C.
The hot set test is a test for measuring the degree of easily changing in shape of an insulation at a high temperature, and even in case of polyethylene that could be used in the past, if it has a low degree of cross-linking, it is easily changed in shape, so that it cannot pass the test. Therefore, it becomes necessary to develop a resin composition including a silane-crosslinked polyethylene that has a higher degree of cross-linking as a main component.